Cation-pi cyclizations initiated by the disrotatory electrocyclic opening of cyclopropanes will be studied as a general method for the synthesis of alicyclic and heterocyclic compounds. The utility of this new synthetic strategy is demonstrated in versatile total syntheses of antimicrobial, antitumor sesquiterpene lactones. The application of this synthetic approach to nitrogen heterocycles is illustrated in a short total synthesis of the neurotoxic spiroalkaloid perhydrohistrionicotoxin.